Process for producing single crystal and silicon crystal wafer

ABSTRACT

The present invention is a method for producing a single crystal in accordance with Czochralski method by flowing an inert gas downward in a chamber  1  of a single crystal-pulling apparatus  11  and surrounding a single crystal  3  pulled from a raw material melt  2  with a gas flow-guide cylinder  4 , wherein when a single crystal within N region outside OSF region generated in a ring shape in the radial direction of the single crystal is pulled, the single crystal within N region is pulled in a condition that flow amount of the inert gas between the single crystal and the gas flow-guide cylinder is 0.6 D(L/min) or more and pressure in the chamber is 0.6 D(hPa) or less, in which D (mm) is a diameter of the single crystal to be pulled. It is preferable that there is used the gas flow-guide cylinder that Fe concentration is 0.05 ppm or less, at least, in a surface thereof. Thereby, there is provided a method for producing a single crystal, wherein in the case that a single crystal is produced by an apparatus having a gas flow-guide cylinder in accordance with CZ method, the single crystal has low defect density and Fe concentration can be suppressed to be 1×10 10  atoms/cm 3  or less even in a peripheral part thereof.

TECHNICAL FIELD

The present invention relates to a method for producing a single crystalthat is used in semiconductor device production, particularly relates toa method for producing by Czochralski method a silicon single crystalwith extremely high quality in which density of grown-in defects issmall and a concentration of heavy metal impurity such as Fe in aperipheral part is reduced.

BACKGROUND ART

It is remarkable that semiconductor integrated circuit devices have beenhighly integrated and therewith developed to be finer. For improvingprocess yield of device production, there is strong requirement forenlargement in size and higher quality of a wafer used as a substrate.Items which relate to crystal quality such as oxygen concentration of asubstrate and heavy metal impurity affect property of a semiconductorintegrated circuit device (See, Ultra Clean Technology Vol. 5 NO 5/6“heavy-metal contamination and oxide-film defects of a silicon wafer”),particularly it has been reported that dielectric breakdown strength ofa gate oxide film of MOS is degraded by heavy-metal contamination suchas Fe, and the like. Moreover, in the case that a silicon single crystalis contaminated by heavy metal, this has a great influence on lifetimeof minority carriers and has possibilities of causing problems inproperty of the semiconductor integrated circuit device.

Moreover, in particular, as an important point for improving processyield in device production in recent years, improvement of device yieldin a peripheral part of a wafer has become a problem. Therefore, it hasbecome important to reduce contamination of heavy metal such as Fe inthe peripheral part of a wafer. As a cause of the heavy-metalcontamination of a single crystal, there is impurity mixed in melt. And,it was recently found that Fe (iron) released from a gas flow-guidecylinder and such adheres to a single crystal during the pulling.

In CZ method, in particular, in the case that a silicon single crystalhaving a large diameter of 200 mm or more is grown, there is frequentlyused an apparatus in which a gas flow-guide cylinder so as to surroundthe single crystal pulled from a raw material melt is disposed. The gasflow-guide cylinder is also important for straightening flow of an inertgas provided in a chamber during the growth and efficiently exhaustingout of the furnace a silicon oxide that evaporates from the melt. As ageneral gas flow-guide cylinder, a carbon material such as a graphitemember is used and disposed to be close to a crystal by a distance inthe range of 10 mm to 200 mm from the crystal or further by a distanceof 10 to 100 mm. Moreover, as material of the gas flow-guide cylinder,high melting point metal such as tungsten or molybdenum is occasionallyused. Furthermore, in the case that an appropriate cooling medium isused, stainless or copper can be used as material of the gas flow-guidecylinder.

However, if a heavy-metal component such as Fe is released from the gasflow-guide cylinder, it adheres to a crystal surface during the growth,and Fe is diffused toward the crystal center from the crystal peripheryalong with the later growth in a cooling process from an ultra hightemperature during the crystal growth to a room temperature,particularly metal contamination is occasionally caused in theperipheral part of the crystal.

As measures for heavy-metal contamination caused by the gas flow-guidecylinder as described above, it has been proposed that a surface of thegas flow-guide cylinder is coated by a high-purity coating film ofpyrolytic graphite that a Fe concentration is suppressed to be very low,or the like (see, International Publication WO 01-81661). By coating asurface of a gas flow-guide cylinder as described above, release of a Fecomponent from the gas flow-guide cylinder can be suppressed and Feconcentration even in a peripheral part of a grown single crystal can besuppressed to be low.

On the other hand, as devices become highly integrated in recent years,it is also demanded to reduce grown-in defects such as FPD, LSTD, andCOP in a wafer. Grown-in defects are defects caused by single crystalgrowth which are induced in a crystal during growth when a siliconsingle crystal is grown by CZ method.

Hereinafter, there will be described relation between a pulling ratewhen a silicon single crystal is grown by CZ method and defects in thesilicon single crystal to be grown. It is known that in the case that agrowth rate V is changed from a high speed to a low speed in thedirection of the crystal axis by a CZ pulling apparatus, a cross-sectionin an axial direction of the single crystal can be obtained as a defectdistribution view as shown in FIG. 8.

V region in FIG. 8 is a region having a number of Vacancies, i.e.concave parts generated by silicon atom shortage, such as holes. Iregion is a region having a number of dislocations or a number of bodiesof excess silicon atoms which are generated by existence ofInterstitial-Si, which is an excess silicon atom. Neutral region (Nregion) having no or little shortage or excess of atoms exists betweenthe V region and the I region. Moreover, defects, which are referred toas OSF (Oxidation Induced Stacking Fault) in the vicinity of a boundaryof the V region, are distributed in a ring shape (OSF ring) when viewedin a cross-section in a vertical direction to crystal growth axis (in asurface of the wafer).

In the case that the growth rate is relatively high, grown-in defectssuch as FPD, LSTD, and COP originated from voids that vacancy-type pointdefects aggregate exist at high density in the entire region in theradial direction of a single crystal and the region that these defectsexist becomes V region. As the growth rate becomes lower, OSF ring isgenerated from the crystal periphery and N region is generated in theoutside (the lower rate side) of the ring. Furthermore, if the growthrate is low, the OSF ring shrinks to the center of the wafer anddisappears and the entire plane thereof becomes N region. If the growthrate is further lower, L/D (Large Dislocation: general designation of“interstitial dislocation loop”, such as LSEPD or LFPD) defects (largedislocation clusters), which are thought to be originated fromdislocation loops that interstitial silicones aggregate, exist at lowdensity and the region that these defects exist becomes I region(occasionally referred to as L/D region).

N region outside the OSF region between the V region and the I regionbecomes a region having low defect density, in which there exist neitherFPD, LSTD, and COP that are originated from vacancies, nor LSEPD andLFPD that are originated from interstitial silicones. In recent days, ithas been found that if the N region is further categorized, as shown inFIG. 8, there are Nv region (a region where vacancies existpredominately) next to the outside of the OSF ring and Ni region (aregion where interstitial silicones exist predominately) next to Iregion. When thermal oxidation treatment is performed, in the Nv region,amount of precipitated oxygen is rich, and in the Ni region, amount ofprecipitated oxygen is little.

In recent years, in CZ method, by setting growth rate of a crystal to below or by setting a structure inside a furnace of the CZ pullingapparatus for gradual cooling of the crystal, it has become possible toproduce a silicon crystal with low defect density in the entire crystal.

For example, there is proposed a method that by controlling thermalhistory during crystal growth, point defects are reduced (See, JapanesePatent Application Laid-open (kokai) No. 9-202684, and No. 7-41383).Moreover, by controlling V/G, which is a ratio of a pulling rate (V) toan axial temperature gradient (G) at a solid-liquid interface in acrystal, it has become possible to produce a crystal that N region isexpanded in the horizontal entire plane (the entire plane of a wafer)(See, Japanese Patent Application Laid-open (kokai) No. 8-330316 and No.11-147786).

In the case that a crystal with low defect density is grown as describedabove, for improving process yield of device production, it is alsoimportant to reduce contamination of heavy metal such as Fe.

However, when a silicon single crystal having low defect density isproduced, even if there is used a gas flow-guide cylinder coated with acoating film that Fe concentration is extremely low as described above,Fe concentration in a peripheral part of the crystal cannot besuppressed to be sufficiently low, and it has been difficult to make1×10¹⁰ atoms/cm³ or less like demands in recent years. Therefore, therehas been a problem that process yield becomes low in the production ofsemiconductor devices after that.

Moreover, as a method for producing a silicon single crystal having lowdefect density with Fe contamination suppressed, there has been proposeda method that raw material is cleaned with fluoric acid and such, and asingle crystal ingot is pulled by a constant rate (a solidifying rate)from a melting raw material. Furthermore, after chunked or grained, thisis cleaned and melted again, and thereafter a silicon single crystal isgrown by controlling V/G (See, Japanese Patent Application Laid-open(kokai) No. 2000-327485). According to such a method, it is supposedthat there can be grown a silicon single crystal having no grown-indefects which Fe concentration is reduced to be 2×10⁹ atoms/cm³ or less.

However, in a method for growing a silicon single crystal wherein Feconcentration is reduced by repeating the cleaning of raw material, themelting, and the pulling as described above, cost significantlyincreases because pulling is performed twice or more. Even if such amethod is used, there is a problem that there cannot be avoided Fecontamination caused by a gas flow-guide cylinder during growth.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention was conceived in view of the aboveproblems. A main object of the present invention is to provide a methodfor producing a single crystal, wherein in the case that a singlecrystal is produced by an apparatus having a gas flow-guide cylinder inaccordance with CZ method, the single crystal has low defect density andFe concentration can be suppressed to be 1×10¹⁰ atoms/cm³ or less evenin a peripheral part thereof.

In order to accomplish the above object, according to the presentinvention, there is provided a method for producing a single crystal inaccordance with Czochralski method by flowing an inert gas downward in achamber of a single crystal-pulling apparatus and surrounding a singlecrystal pulled from a raw material melt with a gas flow-guide cylinder,wherein when a single crystal within N region outside OSF regiongenerated in a ring shape in the radial direction of the single crystalis pulled, the single crystal within N region is pulled in a conditionthat flow amount of the inert gas between the single crystal and the gasflow-guide cylinder is 0.6 D(L/min) or more and pressure in the chamberis 0.6 D(hPa) or less, in which D (mm) is a diameter of the singlecrystal to be pulled.

If the single crystal within N region is pulled in a condition that flowamount of the inert gas between the single crystal and the gasflow-guide cylinder is 0.6 D(L/min) or more and pressure in the chamberis 0.6 D(hPa) or less as described above, metal components such as Fereleased from the gas flow-guide cylinder is induced to exhaust out ofthe chamber with the inert gas, Fe adhering to a surface of a crystalcan be significantly reduced. Therefore, in accordance with such amethod, there can be produced a high-quality single crystal, wherein aswell as a single crystal has low defect density, Fe concentration can besuppressed to be 1×10¹⁰ atoms/cm³ or less in a peripheral part thereofas required in recent years.

In addition, a gas flow-guide cylinder mentioned in the presentinvention is not limited to one for straightening flow of an inert gasbut is used as a generic term of all members disposed so as to surrounda pulled single crystal above the melt surface, such as aheat-insulating member, a heat shield screen, and a cooling cylinderwhich are disposed for controlling the temperature distribution in thefurnace.

Moreover, in the present invention, it is preferable that the singlecrystal to be pulled is a silicon single crystal.

Because a silicon single crystal has been demanded highly and growthusing a gas flow-guide cylinder is frequently performed, the presentinvention becomes particularly effective.

Moreover, the diameter of the single crystal to be pulled is 200 mm ormore.

A silicon single crystal having a diameter of 200 mm or more,particularly 300 mm is produced, and also in the case that a singlecrystal having such a large diameter is grown, it is necessary that Feconcentration is suppressed to be 1×10¹⁰ atoms/cm³ or less in aperipheral part thereof. However, in the case that a single crystalhaving a large diameter is grown, the possibility that the crystal iscontaminated with Fe becomes high because the pulling rate is lower.Accordingly, in accordance with the present invention, by suppressing Fecontamination effectively and growing a single crystal having a largediameter, high-quality single crystals having a large diameter can beproduced at high productivity.

It is preferable that the single crystal within N region is pulled byusing the gas flow-guide cylinder that Fe concentration is 0.05 ppm orless, at least, in a surface thereof.

If a single crystal is pulled according to the present invention byusing the gas flow-guide cylinder that Fe concentration is extremelyreduced, Fe-releasing from a gas flow-guide cylinder can be more reducedand a single crystal with extremely high quality can be obtained.

Furthermore, according to the present invention, a single crystalproduced by the method described above is provided. And, from thissingle crystal, there is provided a silicon single crystal wafer havinga diameter of 200 mm or more produced in accordance with Czochralskimethod, wherein the wafer is occupied by N region outside OSF regiongenerated in a ring shape in the radial direction of a single crystal,and Fe concentration of the entire plane in the radial directionincluding a peripheral part of the wafer is 1×10¹⁰ atoms/cm³ or less.

A singe crystal produced by a method of the present invention becomes asingle crystal with extremely high quality, wherein the single crystalhas low defect density and Fe concentration can be suppressed to be loweven in a peripheral part thereof. In particular, if wafers are producedfrom a silicon single crystal as described above and used as a substratefor semiconductor devices, yield of devices in a peripheral part of thewafer can be improved.

According to the present invention, there can be produced a singlecrystal within N region having low defect density and prevented frombeing Fe-contaminated even in a peripheral part thereof. Moreover, asingle crystal having high uniformity with regard to lifetimedistribution in a plane thereof can be obtained. If semiconductorintegrated circuit devices are produced by using wafers obtained fromsuch a single crystal, process yield can be improved.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a schematic-view showing an example of a singlecrystal-pulling apparatus.

FIG. 2 is a graph showing relation between flow amount of Ar gas and Fecontamination in a peripheral part of a wafer.

FIG. 3 is a graph showing relation between pressure in a chamber and Fecontamination in a peripheral part of a wafer.

FIG. 4 is a Fe concentration map in a plane of a silicon wafer producedin Example 1.

FIG. 5 is a Fe concentration map in a plane of a silicon wafer producedin Example 2.

FIG. 6 is a Fe concentration map in a plane of a silicon wafer producedin Comparative Example 1.

FIG. 7 is a Fe concentration map in a plane of a silicon wafer producedin Comparative Example 2.

FIG. 8 is a view showing relation between growth rate and defectdistribution of a single crystal grown according to CZ method.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a method for producing a silicon single crystal accordingto the present invention will be concretely explained in detail withreference to appended drawings. However, the present invention is notlimited to this.

The present inventors made diligent studies with regard to Fecontamination of a silicon single crystal grown by CZ method, and thefollowing was found. If a crystal growth rate is high, probability of aFe component adhering to a crystal surface is low, and even if thecomponent adheres, Fe contamination can be suppressed because time ofthe component diffusing inside the crystal is short. However, in thecase that a crystal with low defect density whose entire crystal is tobe N region is produced, even if a gas flow-guide cylinder in which Feconcentration is reduced is used, because the growth rate is slow andthermal history at a high temperature is long, particularly in the caseof growing a single crystal having a large diameter of 200 mm or more,probability of the component adhering to the crystal surface is high dueto the growth rate becoming lower, and small amount of adhering Fediffuses inside the crystal and Fe contamination is caused in aperipheral part.

Accordingly, the present inventors thought that even in the case ofgrowing a single crystal having a large diameter and low defect density,Fe adhering to a surface of a single crystal during growth could bereduced by increasing flow amount of a gas, even in a silicon singlecrystal having low defect density with long thermal history,Fe-diffusing amount toward the crystal center would be small and Fecontamination in a peripheral part could be effectively suppressed.

More analysis was performed in detail. It was found that if the singlecrystal within N region is pulled in a condition that flow amount of theinert gas between the single crystal and the gas flow-guide cylinder is0.6 D(L/min) or more and pressure in the chamber is 0.6 D(hPa) or less,in which D (mm) is a diameter of the single crystal to be pulled, thesingle crystal has low defect density and Fe concentration can besuppressed to be 1×10¹⁰ atoms/cm³ or less even in a peripheral partthereof. Accordingly, the present invention has been accomplished.

FIG. 1 shows a schematic view of an example of a single crystal-pullingapparatus that can be suitably used in the present invention. The singlecrystal-pulling apparatus 11 has crucibles (a quartz crucible 5 and agraphite crucible 6) containing a silicon molten liquid (melt) 2 in achamber 1, a heater 7 is disposed around the crucibles 5, 6, andfurthermore, a heat-insulating member 8 is disposed in an outercircumference thereof. Moreover, at a upper part of the apparatus 11,there is provided a gas-introducing duct 9 for introducing an inert gassuch as Ar during growth and a flow-amount adjusting valve 18. And, at abottom part, a gas-exhausting duct 10 is provided.

Above the crucibles 5, 6, a cylindrical gas flow-guide cylinder 4surrounding a pulled single crystal 3 is disposed, and furthermore anouter insulating member 14 having a ring shape is provided at a bottomthereof.

In addition, with regard to the gas flow-guide cylinder 4 and the outerinsulating member 14 that are used in the present invention, it ispreferable that ones having as small heavy-metal components like Fe aspossible are used. In particular, it is preferable that ones in which Feconcentration is 0.05 ppm or less, at least, in a surface thereof areused. For example, the gas flow-guide cylinder on which a pyrolyticgraphite coating film with high purity having a Fe concentration of 0.05ppm or less is formed can be suitably used.

If such a pulling apparatus 11 is used, difference between temperaturegradient Gc [° C./cm] in the central part of the crystal and temperaturegradient Ge [° C./cm] in a peripheral part of the crystal becomes small.For example, temperature in the furnace can also be controlled so thatthe temperature gradient in the peripheral part of the crystal is lowerthan that in the crystal center.

In addition, an inner heat-insulating member may be also provided insidethe gas flow-guide cylinder 4, or MCZ method may be used that a magnetis placed outside the chamber 1 and magnetic field is applied to asilicon melt 2 in a horizontal direction, a vertical direction, or thelike.

When a single crystal is grown, a seed crystal 13 is held by a holder 12and silicon polycrystal material with high purity is heated at a meltingpoint (about 1420° C.) or more and melted in the crucibles 5, 6. Bywinding a wire 15 off, a tip end of the seed crystal 13 is contacted orimmersed with about the central part of the surface of the melt 2. Afterthen, along with rotating the crucibles 5, 6 and rotating the wire 15,the wire is slowly rewinded. Thereby, growth of a single crystalfollowing the seed crystal 13 is initiated, and thereafter a singlecrystal ingot 3 having about a columnar shape can be pulled bycontrolling the pulling rate and the temperature appropriately.

In addition, during the growth, an inert gas such as Ar is introducedfrom the gas-introducing duct 9, flowed downward in the chamber 1,passed between the pulled single crystal 3 and the gas flow-guidecylinder 4, and thereafter exhausted with a vacuum pump 17 through theexhaust duct 10. At this time, flow amount of the introduced gas can beregulated by adjusting the valve 18, and pressure in the furnace can beregulated by adjusting open degree of a valve 16.

For pulling a single crystal having low defect density and particularlywithin N region outside OSF region generated in a ring shape in theradial direction thereof, for example, a single crystal may be pulled bycontrolling to be a growth rate (a pulling rate) between the growth rateof the boundary that OSF region generated in a ring shape disappears inthe case of gradually reducing a growth rate from a high rate to a lowrate and the growth rate of the boundary that interstitial dislocationloops are generated in the case of more reducing a growth rategradually.

If a single crystal is grown as described above, a single crystal withinN region outside OSF region can be pulled. However, in the case that asingle crystal is grown as described above, there is tendency that apulling rate is relatively slow and thermal history at a hightemperature becomes long. Therefore, if a Fe component released from thegas flow-guide cylinder and such adheres to a surface of the singlecrystal during growth, Fe is diffused inward in the crystal and Fecontamination in a peripheral part of the single crystal is easilycaused.

Accordingly, in the present invention, the above-described singlecrystal within N region is pulled in a condition that flow amount of theinert gas between the single crystal and the gas flow-guide cylinder is0.6 D(L/min) or more and pressure in the chamber is 0.6 D(hPa) or less,in which D (mm) is a diameter of the single crystal to be pulled. Ifduring the growth, for example, Ar gas is introduced with flow amountaccording to a diameter of the crystal and flowed downward andadditionally adjusted to have a pressure according to a diameter of thecrystal as described above, even if a metal component such as Fe isreleased from the gas flow-guide cylinder, the component with the Ar gasis induced to exhaust out of the furnace. Therefore, Fe and suchadhering to the single crystal surface during growth is greatly reduced.Even if thermal history at a high temperature is long, Fe diffusinginward in the crystal can be prevented. As a result, there can beproduced a silicon single crystal wherein Fe concentration is extremelyreduced even in a peripheral part of the crystal.

FIG. 2 shows relation between flow amount of Ar gas in growth of asilicon single crystal within N region and Fe concentration (averagevalues of detected values) in a peripheral part of a silicon waferproduced from the grown single crystal.

Using a pulling apparatus such as FIG. 1, the present inventors adjustedthe valve of the vacuum pump to set the pressure in the chamber to beconstant (150 hPa), variously altered the flow amount of Ar gas floweddownward in the chamber, and grew a silicon single crystal within Nregion (having a diameter of 300 mm). From each of the grown singlecrystals, through steps of slicing, chamfering, lapping,mirror-polishing, and such, silicon mirror wafers were produced. Then,Fe concentration was measured in a peripheral part (10 mm from anextreme periphery, 10 points) of the obtained silicon wafer.

As shown in FIG. 2, it is found that as flow amount of Ar gas is larger,Fe concentration tends to decrease. In particular, by performing growthwith setting the flow amount of Ar gas to be 0.6 D=180 (L/min) or more,Fe concentration can be suppressed to be 1×10¹⁰ atoms/cm³ or less.

Furthermore, FIG. 3 shows relation between pressure change in a chamber(in a furnace) and Fe concentration in a peripheral part of the crystalwhen flow amount of Ar gas is constant, 120 L/min (corresponding to 0.6D), in the case of growing a singe crystal within N region (a diameterD=200 mm).

As shown in FIG. 3, it is found that as pressure of Ar gas in a chamberis smaller, Fe concentration is lower. In particular, Fe concentrationof 1×10¹⁰ atoms/cm³ or less can be attained if the pressure in thechamber is 0.6 D=120 (hPa) or less.

A silicon single crystal produced according to the present invention asdescribed above can be a high-quality single crystal that as well as thecrystal is within N region in which grown-in defects does not exist orare extremely reduced, Fe concentration of the entire plane in theradial direction including a peripheral part of the single crystal canbe suppressed to be 1×10¹⁰ atoms/cm³ or less. Therefore, ifsemiconductor device production is performed by using this as asubstrate, yield in a peripheral part of the substrate is improved, andreduction of production cost as well as improvement of process yield canbe accomplished.

In addition, it is desirable that the flow amount of an inert gas islarger in the present invention because Fe concentration can be reduced.However, it is not preferable that the flow amount is too large becausegas is wasteful, the grown single crystal is shaked, the melt surface isruffled, and so forth. Therefore, preferably it is about 10 D or less.Moreover, it is desirable that the pressure is lower because Feconcentration can be reduced. However, if the pressure is too low,degradation of the used quartz crucible is intensified. Therefore, it ispreferable that the pressure is 0.01 hPa or more, more preferably, 0.1hPa or more.

Hereinafter, according to the present invention, Examples andComparative Example will be explained.

EXAMPLE 1

In a single crystal-pulling apparatus as shown in FIG. 1, after 120 kgof polycrystalline silicon was charged in a quartz crucible having adiameter 56 cm and melted, a seed crystal having a <100> plane wasimmersed in the silicon melt, and through a necking step, there wasgrown a silicon single crystal having a diameter of D=200 mm within Nregion which is doped with boron so that the resistivity thereof becomes10 Ωcm. As a gas flow-guide cylinder, there was used a body consistingof graphite material, on which a pyrolytic graphite coating film of highpurity having a Fe concentration of 0.05 ppm or less was formed by CVDmethod.

In addition, the growth rate was about 0.5 mm/min, Ar gas was floweddownward along with adjusting the flow amount to 140 L/min(corresponding to 0.7 D) and the pressure in the furnace to 120 hPa(corresponding to 0.6 D) during the growth.

From the grown silicon single crystal, silicon wafers were producedthrough such general processes required for producing industriallysilicon wafers as cylindrically grinding, slicing, lapping, andpolishing. Fe distribution in a plane of the wafer was measured.

In addition, the Fe concentration measurement of the wafer was performedby SPV method (Surface Photo-voltage Method). Fe solid-solubilized inthe boron-doped silicon single crystal was stabilized in a FeB form bybinding to boron, which is a dopant, in a room temperature. Bindingenergy of FeB is about 0.68 eV and most of FeB is dissociated into Feiat about 200° C. Fei form a deep level and therefore functions as arecombination center of minority carriers and depresses diffusion lengthof minority carriers. That is, the diffusion length of minority carriersis long before heat treatment at about 200° C., however because Feifunctions as recombination centers of minority carriers after the heattreatment, the diffusion length of minority carriers is shortened. Feconcentration can be measured by measuring difference between thediffusion length.

As a result, a Fe concentration map in a plane as shown in FIG. 4 couldbe obtained. It was confirmed that Fe concentration was 1×10¹⁰ atoms/cm³or less in the entire surface of the wafer, particularly there was notsuch Fe contamination as Fe concentration being more than 1×10¹⁰atoms/cm³ even in a peripheral part thereof.

EXAMPLE 2

A silicon single crystal was grown by a same manner as Example 1 exceptfor the pressure in the chamber being 80 hpa (corresponding to 0.4 D).And there was measured Fe distribution in the peripheral part of thewafer produced from the obtained silicon single crystal. The measurementresult shown in FIG. 5 was obtained and it was confirmed that there wasnot such Fe contamination as a Fe concentration of more than 1×10¹⁰atoms/cm³ even in a peripheral part thereof.

COMPARATIVE EXAMPLE 1, 2

A conventional crystal (a diameter of 200 mm) having many grown-indefects (V region) by using the same single crystal-producing apparatusas Example 1 was produced on the following condition. The flow amount ofAr gas was adjusted to 80 L/min (corresponding to 0.4 D), the pressurein the furnace to 300 hPa (corresponding to 1.5 D), and the growth rateto 1.0 mm/min (Comparative Example 1). The other conditions were thesame as Example 1.

Moreover, a silicon single crystal having low defect density was grownat a growth rate of 0.5 mm/min under the same flow amount of Ar gas (80L/min) and the same pressure (300 hpa) as described above (ComparativeExample 2).

A silicon wafer was produced from each of the grown single crystals andFe distribution in a plane of the wafer was measured as described above.

In the wafer of Comparative Example 1, there was no Fe contaminationeven in a peripheral part thereof as shown in FIG. 6, however there weremany grown-in defects in the entire plane of the wafer. In addition, thereason why there was no Fe contamination in a peripheral part thereof isthought that the pulling rate was high, Fe adhering to a surface of thecrystal was small, and Fe diffusing inward in the crystal was small.

On the other hand, the wafer obtained in Comparative Example 2 had lowdefect density, however, it was confirmed that as shown in FIG. 7, Feconcentration near 10-30 mm from a periphery of the wafer was more than1×10¹¹ atoms/cm³, and also a part with further more than 1×10¹²atoms/cm³ was shown, and Fe contamination occurred. This is thoughtbecause flow amount of Ar gas was smaller than that in Example 1 andmuch Fe adhered to a surface of the crystal during growth and a part ofthem was diffused inside the crystal.

In addition, the present invention is not limited to the embodimentsdescribed above. The above-described embodiments are merely examples,and those having the substantially same constitution as that describedin the appended claims and providing the similar working effects areincluded in the scope of the present invention.

For example, the used single crystal-pulling apparatus is not limited tothe one in FIG. 1 and there can be used all of the pulling apparatusthat have a gas flow-guide cylinder and can be grown a single crystal sothat the entire crystal becomes N region. Also, the inert gas is notlimited to Ar, and the present invention can be applied in the case thatother gases such as helium and nitrogen are provided.

1-6. (canceled)
 7. A method for producing a single crystal in accordancewith Czochralski method by flowing an inert gas downward in a chamber ofa single crystal-pulling apparatus and surrounding a single crystalpulled from a raw material melt with a gas flow-guide cylinder, whereinwhen a single crystal within N region outside OSF region generated in aring shape in the radial direction of the single crystal is pulled, thesingle crystal within N region is pulled in a condition that flow amountof the inert gas between the single crystal and the gas flow-guidecylinder is 0.6 D(L/min) or more and pressure in the chamber is 0.6D(hPa) or less, in which D (mm) is a diameter of the single crystal tobe pulled.
 8. The method for producing a single crystal according toclaim 7, wherein the single crystal to be pulled is a silicon singlecrystal.
 9. The method for producing a single crystal according to claim7, wherein the diameter of the single crystal to be pulled is 200 mm ormore.
 10. The method for producing a single crystal according to claim8, wherein the diameter of the single crystal to be pulled is 200 mm ormore.
 11. The method for producing a single crystal according to claim7, wherein the single crystal within N region is pulled by using the gasflow-guide cylinder that Fe concentration is 0.05 ppm or less, at least,in a surface thereof.
 12. The method for producing a single crystalaccording to claim 8, wherein the single crystal within N region ispulled by using the gas flow-guide cylinder that Fe concentration is0.05 ppm or less, at least, in a surface thereof.
 13. The method forproducing a single crystal according to claim 9, wherein the singlecrystal within N region is pulled by using the gas flow-guide cylinderthat Fe concentration is 0.05 ppm or less, at least, in a surfacethereof.
 14. The method for producing a single crystal according toclaim 10, wherein the single crystal within N region is pulled by usingthe gas flow-guide cylinder that Fe concentration is 0.05 ppm or less,at least, in a surface thereof.
 15. A single crystal produced by themethod according to claim
 7. 16. A single crystal produced by the methodaccording to claim
 8. 17. A single crystal produced by the methodaccording to claim
 9. 18. A single crystal produced by the methodaccording to claim
 10. 19. A single crystal produced by the methodaccording to claim
 11. 20. A single crystal produced by the methodaccording to claim
 12. 21. A single crystal produced by the methodaccording to claim
 13. 22. A single crystal produced by the methodaccording to claim
 14. 23. A silicon single crystal wafer having adiameter of 200 mm or more produced in accordance with Czochralskimethod, wherein the wafer is occupied by N region outside OSF regiongenerated in a ring shape in the radial direction of a single crystal,and Fe concentration of the entire plane in the radial directionincluding a peripheral part of the wafer is 1×10¹⁰ atoms/cm³ or less.